Final answer:
The instrument with 13 strings stretched over a soundboard and played with ivory plectra is a Koto, an option not presented in the multiple choices given.
Step-by-step explanation:
The instrument described with 13 nylon or silk strings stretched over a hollow rectangular soundboard, played with ivory plectra on the thumb, index finger, and middle finger is known as a Koto. A koto typically measures about six feet in length and has movable bridges for each string to adjust the pitch. None of the options A) Harp, B) Violin, C) Flute or D) Saxophone correctly identifies this instrument, as they do not match the provided description. The correct answer to this question would be 'Koto', which is not listed among the options.